The present invention relates to water irrigation apparatus, and also to a pipe construction particularly useful in such apparatus. The invention is especially useful in mobile irrigation systems of the pivot, linear or side-roll type, and is therefore described below with respect to such application.
Pivot, linear and side-roll water irrigation systems include a water supply pipe which is supported between large spans thereof as the pipe is moved over the ground to be irrigated. The water supply pipe may be supported by wheels or towers, and the spans between the wheels or towers are typically from 40 to 120 feet. These spans usually include additional external supporting structures, such as bridges, beams, girders, channels, trusses or wires, but such reinforcement structures add very substantially to the overall cost of the apparatus. On the other hand, increasing the length of the spans can decrease the overall cost, but the span length is very substantially limited by the weight of the water distribution pipe plus the weight of the water carried by it.
For example, with respect to side-roll systems having a pipe supported between two mobile wheels, the present practice is to move the water supply pipe to each location after having been drained of water, in order to reduce its weight; when at the desired location to fill it and to effect the water distribution; and after the water supply pipe has been emptied, to move it to the next location to tbe irrigated. This is because of the weight of the pipe when filled with water. Thus the weight of the water in the water supply pipe is many times the weight of the pipe itself, being about six times the weight of a three-inch aluminum pipe and about ten times the weight of a six-inch aluminum pipe. A forty-foot span of aluminum pipe filled with water weights about 144 pounds for a three-inch pipe, 247 pounds for a four-inch pipe, 378 pounds for a five-inch pipe, and 540 pounds for a six-inch pipe. If such a pipe when filled with water is moved across the field, the pipe will undergo a substantial deflection which, even though not beyond the yielding point, nevertheless can cause early failure of the pipe. This is because rotating the pipe while so deflected by the weight of the pipe will produce frequent deflection reversals, producing torsional stresses as well as deflection stresses, which will accelerate the chance of rupture of the pipe.